The present invention relates generally to shot peening, and, more specifically, to laser shock peening.
In conventional shot peening, small balls are fired against the surface of a metallic workpiece or target to create plastic deformation thereat and a corresponding residual compressive stress. The residual stress improves the useful fatigue life of the workpiece when it is used in a high stress application.
Laser shock peening is being developed to provide improvements in forming the residual compressive stress in the workpiece surface. A laser is operated, typically, in a pulse mode for directing laser pulses against the workpiece surface, which workpiece surface typically has a light absorbing ablative coating confined by a thin layer of water for example. The laser pulse vaporizes the coating in a small explosion that is confined by the water developing an instantaneous pressure pulse. The resulting pressure pulse plastically deforms the workpiece surface to generate residual compressive stress therein.
A major control parameter in this process is the fluence of the laser beam that is defined as the energy per unit area of the beam. Fluence at the workpiece must be high for effecting laser shock peening, but fluence in the resonator or oscillator producing the laser beam must be low to prevent optical damage.
This control may be accomplished by using a low energy oscillator followed in turn by several rod amplifiers in which the fluence is increased in turn and protects the laser equipment from thermal damage. This configuration, however, requires many laser heads or lasing gain media, optical pumping flash lamps, power supplies, and related equipment that increases the complexity of the system and susceptibility to failure during operation.
Another useful laser system having few components is disclosed in U.S. Pat. No. 5,730,811-Azad et al. In this configuration, a large Q-switched, cavity-dumped, laser oscillator produces a significant fraction of the required energy for laser shock peening in a single-head resonator, either with or without an additional amplifier cooperating therewith. This high energy laser requires a suitably large oscillator and cooperating optical elements and Pockels cell for effecting the Q-switching and cavity-dumping. The large elements reduce the fluence in the oscillator for preventing thermal damage thereto from the high energy laser beam created therein.
The major component of the Pockels cell is a crystal of potassium dideuterium phosphate, also referred to as KD*P. For the energy levels required for laser shock peening, a crystal of about 5 cm in diameter is required. Homogeneity of the electric field in such a large diameter crystal requires a relatively long length thereof of about 6 cm. Accordingly, even the slightest absorption in the crystalline bulk constitutes a significant loss in the laser oscillator.
Furthermore, such absorption in the crystal creates optical distortion that causes the oscillator to produce a highly distorted output beam. This distortion problem may be mitigated by using a highly deuterated KD*P crystal greater than about 99%. Such a large Pockels cell crystal, however, is quite expensive, and therefore impractical to use in a commercial laser shock peening system.
Accordingly, it is desired to provide a laser shock peening apparatus with few components that does not require a large Pockels cell for producing a high fluence laser beam at the workpiece.